1. Field of the Invention
The present invention relates to cooling systems and, more particularly, to cooling systems for electrical apparatus. Generally, the present invention provides a cooling system for apparatus powered by electricity, that generates a substantial amount of heat during operation, and the heat must be dissipated to avoid failure of electrical and/or electronic components, such as semiconductor devices and integrated circuits, comprising the electrical apparatus. Specifically, one embodiment of the present invention provides a cooling system preferably employing water jet impinged in a partial vacuum on a heat sink thermally coupled to electrical apparatus, and the attendant phase change of the water to steam at the reduced boiling point due to the subatmospheric pressure, to remove a substantial amount of waste heat to prevent failure of the electrical apparatus.
2. Description of the Prior Art
Cooling is an important process associated with operation of high-density electronic devices. Existing waste heat removal technology is limited to approximately 100 W/cm2.
In the next ten years, the power density of high-power electronics is expected to increase and generate waste heat that will exceed 1,000 W/cm2. Thermal management technology capable of removing waste heat of 1,000 W/cm2 produced by advanced power electronic devices is needed.
For example, the U.S. Department of Navy has reported that the cooling requirements are expected to increase at least an order of magnitude during the next decade. As stated in xe2x80x9cNext Generation Navy Thermal Management Program,xe2x80x9d CARDIVNSWC-TR-82-2002/12, by Michael Kuszewski and Mark Zerby, Naval Surface Warfare Center:
xe2x80x9cIt is expected that heat fluxes for new technologies such as Advanced Radar will exceed 1000 W/cm2, and some advanced weapons may be higher. These heat fluxes are expected to be present by the end of this decade. Heat fluxes are growing so fast in the electronics arena that even Intel, who has been designing its Thermal Management Systems to handle less than 100 W/cm2, has extrapolated its increase of heat flux to reach 1000 W/cm2 before the end of this decade.xe2x80x9d
Accordingly, the U.S. Navy recently published RFQ N03-T022 Acquisition Program: DD(X); CVN(X) having the:
xe2x80x9cOBJECTIVE: To develop advanced thermal management technologies to improve high flux waste heat removal by a factor of 10xc3x97over existing technologies in electronic devices.xe2x80x9d
Also,
xe2x80x9cThe proposed solution must be able to keep the semiconductor junction below 125 F [sic, 125 C] . . . xe2x80x9d
Spray cooling with water at atmospheric pressure is a known cooling technique to remove heat from electronics relatively efficiently. See, Kuszewski and Zerby, supra. In situations where cooling very hot surfaces or protecting sensitive surfaces from overheating is important, then the most effective previously known technique available is direct impact by impingement jets (not necessarily sprays) at atmospheric pressure. The reverse side of a mounting plate, on which the electronic devices are disposed, is sprayed by high velocity impinging jets of water. The heat generated by the electronics is removed at constant temperature by the liquid vapor phase of the water.
The heat transfer processes involved in water sprays impinging on hot surfaces at atmospheric pressure have been studied by, among others, Bernardin J D, and Mudawar I, xe2x80x9cFilm boiling heat transfer of droplet streams and sprays,xe2x80x9d Intl. J. Heat Mass Transfer, 40 (11), 2579-2593 (1997). Rockwell has also published a paper that reports having achieved removal of 1,000 W/cm2 using a water jet plus boiling at atmospheric pressure. However, Rockwell was only able to cool a very small area (unspecified).
Additionally, it has been reported that heat pipes using water have removed 550 W/cm2 over small areas to provide waste heat removal from hot electronic components. See, Kuszewski and Zerby, supra. The heat pipes were used to transport and spread waste heat. The heat pipes were operated at 70xc2x0 C. (at pressure below one atmosphere) to accomplish a change in phase. However, heat pipes are passive; that is, they employ the capillary action of water in conjunction with wicks to transport water. Consequently, heat pipes cannot remove large amounts of waste heat on the order of 1,000 W/cm2.
The challenge presented by the need to conduct waste heat from electronic devices efficiently and to provide removal of waste heat on the order of 1,000 W/cm2 at a rate that will maintain the operating temperature of electronic devices at or below 125xc2x0 C. is imposing. The 125xc2x0 C. limit requires efficient heat transfer to sink heat away from the electronic apparatus. The high heat flux (1,000 W/cm2) further requires an effective heat removal process to maintain the operating temperature of electronic devices at or below the 125xc2x0 C. limit.
It would therefore be desirable to provide removal of waste heat from electronic devices to maintain the operating temperature of electronic devices at or below 125xc2x0 C. It would also be desirable to remove waste heat at a rate to prevent the operating temperature of electronic devices from exceeding the 125xc2x0 C. limit. Furthermore, it would be desirable to achieve these objectives for electrical apparatus that generates waste heat on the order of 1,000 W/cm2. Additionally, it would be desirable to use water as the coolant.
One embodiment of the present invention provides a cooling system for thermally conducting and removing high heat flux waste heat using water. The cooling system in accordance with one embodiment of the present invention employs jet impingement of water at a predetermined reduced pressure to improve high heat flux waste heat removal by a factor of ten times over present cooling techniques. One embodiment of the cooling system in accordance with the present invention is especially suitable to the challenge of removing high heat flux waste heat resulting from operation of power electronics given the severe limitation on the maximum operating temperature allowable for electronic devices.
One preferred embodiment of the cooling system in accordance with the present invention provides a heat transfer plate consisting of copper, aluminum, silver, or another suitable thermally conductive material, such as beryllium oxide ceramic, boron nitride, aluminum nitride ceramic, or diamond, with high tensile strength, thermally coupled, for example, in thermal contact with, the electrical apparatus. The heat transfer plate also serves as a structural component of a circulation subsystem that contains the water.
The circulation subsystem comprises compressors, condensers, and pumps to circulate water to the location where the water is jet impinged on the heat transfer plate, as well as to remove and condense steam that is generated at that location and, preferably, recalculate the condensate. Unlike heat pipes, which are passive in that they employ the capillary action of water in conjunction with wicks to transport water, the circulation subsystem comprises compressors, condensers, and pumps to transport water. Consequently, the circulation subsystem can remove large amounts of waste heat compared to heat pipes.
Impinging jets deliver copious amounts of water to the hot surface of the heat transfer plate opposite the side on which the electronic apparatus is disposed preferably in thermal contact with the heat transfer plate. In a preferred embodiment of the present invention, jet impingement of water at a predetermined subatmospheric pressure is employed.
Jet impingement of water is provided on the heat transfer plate at a reduced pressure ranging from partial vacuum to approximately total vacuum. Preferably, the water is jet impinged in a partial vacuum, wherein the temperature associated with the phase change of boiling is lowered to enhance the cooling efficacy.
With the operating temperature of electronic devices required to be at 125xc2x0 C. or below, and the temperature of the water at approximately 18.3xc2x0 C., there is a large temperature differential and phase change as the water transforms to steam to effect heat removal and cooling as the steam is circulated by the circulation subsystem away from the heat transfer plate. In addition, the jet impingement maintains steam bubbles associated with boiling small, resulting in more effective heat transfer. Finally, use of water as a coolant is compatible with cooling systems aboard ships operated by the U.S. Navy, thereby satisfying the apparent desirability and advantage to integrate the cooling system in accordance with the embodiments of the present invention with other cooling systems on a ship (for example, air conditioning systems).
The foregoing and other objects, features, and advantages of the present invention will become more readily apparent from the following detailed description of various embodiments, which proceeds with reference to the accompanying drawing.